1. Field of the Invention
This invention relates to a valve seat adapted to be embedded in a ball valve, which fulfills an effective function of relieving the increased pressure occurring anomalously in a cavity formed between the ball and the valve casing of the ball valve.
2. Description of the Prior Art
In a ball valve wherein the ball having a through passage is securely encased in the valve casing by the use of a pair of valve seats, there is formed a closed narrow cavity among the ball, the valve casing and the paired valve seats. There is a case where a fluid flowing through the ball valve intrudes into the cavity from various sources and the fluid remaining within the closed cavity is expanded or vaporized to excessively increase the pressure within the cavity due to a high temperature fluid which flows through the ball valve or the heat involved from an environmental cause, thereby bringing about deformation or destruction of the valve seat, the valve casing or the ball.
To avoid this situation, there has been proposed a valve seat wherein the lip portion, which comes into partial contact with the ball in the ball valve, is remarkably reduced in thickness to form an easy-to-flex portion for relieving a pressure which increases anomalously in the cavity between the ball and the valve casing. (Refer to Japanese Utility Model Public Disclosure No. 82360/1981, for example.)
Another valve seat of this type which relieves the pressure increased in the cavity by bending the flexible lip portion and permitting the entire seat to be subject to torsional motion has been disclosed in U.S. Pat. No. 4,236,691.
Inevitably, the former valve seat becomes weak in structure due to the thin-walled portions provided on the lip portion. That is, the lip portion of the valve seat, which is in sealing contact with the ball, is partially made thin, meaning that the valve seat is easily deformed even under a light load to reduce the sealing efficiency and is liable to sustain plastic deformation, resulting in requiring an early replacement of the seat in the ball valve.
The latter valve seat cannot ensure sufficient strength in structure because the pressure relieving function is carried out by permitting the entire seat body to be twisted and causing the lip portion thereof to be bent by force of the increased pressure in the cavity of the ball valve. Since the entire seat body suffers large torsional stress at the time the pressure in the cavity increases excessively and the lip portion is necessarily made thin to be endowed with elasticity, this valve seat has less durability and cannot assure the complete sealing for a long time.
Thus, the conventional valve seats including the aforementioned valve seats are disadvantageous in that they are complicated and weak in structure and poor in durability and cannot offer a reliable pressure relief function.